The development of self-tailing winches over the last few years has been rapid. Characteristically a winch drum has adjacent one end a tailing channel. After line has been wound a few turns around the winch drum it is placed in the tailing channel. When the winch is first operated the tailing channel grips the line thereby enabling, once tension comes on the line, a firm coil of rope to be set up on the winch drum which thereupon takes up the majority of the driving load by frictional interaction with the line.
An example of such a construction is seen in U.S. Pat. No. 3,968,953 of Guangorena.
It will be seen that there are in general two problems with this type of construction. Firstly, there is the problem of accommodating the channel to receiving a range of thicknesses of line, which may be required to be handled by a given model of winch. In the U.S. Patent mentioned above an attempt has been made to meet this problem by spring-loading the jaws so that they can move apart. The degree of movement will depend partly on the pull being exerted on the line and partly in accordance with the diameter of that line. However, apart from the complexity of construction which is involved, it is essential to set the springs correctly. An excessive spring pressure will cause rejection of the line or, if it is received, difficulty in tripping from the jaw; and a weak spring pressure will mean not enough pulling action is exerted on the line.
Most self-tailing channels designed so far have had teeth in the form of ridges or ribs on both of the jaws which define the channel and these have always been opposed directly to each other. The teeth are regarded as compressive and gripping in their action, exerting a squeezing effect of the rope and they are frequently unidirectional so as that there is a greater resistance to slippage of the line in one tangential direction than in the other tangential direction.
The concept in the prior art of wedging the line between opposed jaws (with or without springs on the jaws) means that the degree to which the line is wedged is very largely although not entirely independent of the load to which the line is being subjected. There is a slight degree of dependence because clearly if a very strong load is imposed on the line it will tend to get squeezed further in between the coverging jaws.
In U.S. Pat. No. 3,968,953 one of the jaws is without teeth, being a smooth plate. This expedient has also been adopted (in a different construction and for a different reason) in our co-pending U.S. application Ser. No. 797,251 filed 16th May 1977.
However, even in the type of construction where one of the jaws is without teeth the basic object of the jaws remains that the smooth jaw will provide a reaction surface between which and the toothed jaw the line is to be compressed and thereby gripped by the teeth of the toothed jaw.
Lastly in the prior art there have been examples of pulley-like devices, see for example Newell U.S. Pat. No. 3,343,809, where the jaws of a line or chain pulling device have been mutually shaped and adapted so as to conform them more or less exactly to the configuration of the line or chain being received in them. The object of such constructions is to achieve an entirely positive, sprocket-like, drive between the line or chain and the pulley upon which it acts. These are not self-tailing devices and the underlying intention is not that the "teeth" shall be staggered but rather that the conformation of each jaw shall complement that of the other in such a way as to adapt to the configuration of the line or chain. Positive drive will not be achieved unless the correct line or chain is used with the device.